The present invention relates generally to systems and methods for imaging tissue using magnetic resonance imaging, and more particularly to systems and methods for performing thermal-sensitive imaging of both fat and muscle tissue using focused magnetic resonance imaging.
A number of methods have been proposed for directing heat to a target tissue region within a patient, such as a cancerous or benign tumor, to necrose or otherwise treat the tissue region with thermal energy. For example, a piezoelectric transducer located outside the patient""s body may be used to focus high intensity acoustic waves, such as ultrasonic waves (acoustic waves with a frequency greater than about twenty kilohertz (20 kHz), and more typically between one and five Megahertz (1-5 MHz)), at an internal tissue region of a patient to therapeutically treat the tissue region. The ultrasonic waves may be used to ablate a tumor, thereby obviating the need for invasive surgery. In an alternative method, laser fibers may be introduced into the patient""s body from an entry site that are used to guide coherent optical heat sources to an internal tissue region.
During such procedures, it is often desirable to image the tissues being treated. For example, ultrasound imaging systems may be used for imaging, as well as for generating therapeutic ultrasound waves. Alternatively, magnetic resonance imaging (or xe2x80x9cMRIxe2x80x9d) may be used instead of ultrasound imaging, as MRI provides excellent quality images of tissue, and is not limited to xe2x80x9cwindowsxe2x80x9d that exclude bone or other structures that may interfere with or otherwise limit ultrasound imaging.
An MRI system may be used to plan a procedure, for example, before surgery or a minimally invasive procedure, such as an ultrasound ablation procedure. A patient may initially be scanned in an MRI system to locate a target tissue region and/or to plan a trajectory between an entry point and the tissue region in preparation for a procedure. Such preparation may be particularly useful because a tumor may be more visible in an magnetic resonance (xe2x80x9cMRxe2x80x9d) image than using direct examination. Due to differences in relaxation times of tumorous and other tissue, MRI images may provide a contrast not available using direct visualization, particularly since tumorous tissue may visually appear similar to normal tissue, or the field of view may be obscured, for example, by blood.
Once the target tissue region has been identified, MRI may be used during the procedure, for example, to image the tissue region and/or to guide the trajectory of an external ultrasound beam to a target tissue region being treated, or to guide laser energy. In addition, an MRI system may be used to monitor the temperature of the tissue region during the procedure, for example, to ensure that only the target tissue region is necrosed during an ablation procedure without damaging surrounding healthy tissue. Generally, this involves using a separate scanning sequence that provides temperature information, in addition, to a scanning sequence that provides tissue information.
For example, before applying sufficient energy to necrose tissue, a lower level of energy may be directed towards the target tissue region, generally in a pulsed or oscillating manner to minimize the effect of thermal diffusion. As the tissue region is heated, a temperature-sensitive magnetic resonance (xe2x80x9cMRxe2x80x9d) pulse sequence may be used to acquire a temperature xe2x80x9cmapxe2x80x9d to ensure that the energy is applied to the target tissue region and not to the surrounding healthy tissue. The imaging system may also be used in a separate scan sequence to create an image of the tissue intended to be destroyed, and then the two images may be superimposed upon one another to identify the location of the energy relative to the target tissue region.
The placement of the energy may then be adjusted to direct the energy more accurately towards the target tissue region. For example, a focal zone of ultrasonic.energy emitted by an ultrasound transducer may be moved by mechanically adjusting the position of the transducer relative to the patient""s body. Alternatively, the focal zone may be moved electronically. e.g., by controlling a phase component and/or relative amplitude of drive signals to the transducer elements, or a combination of mechanical and electronic positioning may be used, as is known in the art.
MRI systems exploit the property that free unpaired spinning protons in the nucleus of a molecule of a specific tissue, such as hydrogen molecules, align themselves in a magnetic field such that their axes precess about the magnetic field. Such unpaired protons have non-zero xe2x80x9cspinxe2x80x9d and consequently behave like a small magnetic dipole. The net sum of the population of dipoles results in a bulk magnetization vector M that is aligned with a static magnetic field B0, shown in FIG. 1 in a reference frame Xxe2x80x2Yxe2x80x2Zxe2x80x2, and rotating about the static magnetic field axis at a frequency equal to the precession of the spins (the Larmor frequency).
The magnetic dipoles forming the net magnetization vector M ordinarily are aligned with the applied magnetic field. However, these magnetic dipoles have an excited state that opposes this applied magnetic field. Pulses resulting from an RF excitation at the Larmor frequency will cause the magnetic dipoles to transition from the aligned state to the opposing state. An MR imaging device uses a radio frequency (RF) transmitter to xe2x80x9cflipxe2x80x9d the magnetic dipoles into the excited state by transmitting RF energy at the Larmor frequency. For example, a one hundred eight degree (180xc2x0) pulse from the RF transmitter will rotate or xe2x80x9cflipxe2x80x9d the magnetization vector M down to align along the xe2x88x92Zxe2x80x2 axis. This behavior is generic to any orientation of the magnetization vector.
Regardless of the flip angle excited by the RF transmitter, it is possible, by applying a magnetic field gradient, to selectively choose spins for excitation. This selection is necessary for imaging of a tissue structure. A one-dimensional linear magnetic field gradient, conventionally denoted in the Z direction, is applied during the RF excitation pulse. Because of the linear gradient, only spins located in a particular slice or plane through the patient will respond to a given RF pulse.
FIG. 2 illustrates the resulting net magnetization vector M after application of a ninety degree (90xc2x0) pulse. This vector aligns with the Y2 axis and thus is entirely in the transverse Xxe2x80x2Yxe2x80x2 plane. Two time constants, T1 and T2, govern the relaxation of this perturbed or excited magnetic field vector back to the equilibrium state of FIG. 1. T1 relates to the time necessary for the decay in the longitudinal component of the excited magnetization vector. T2 relates to the time necessary for the decay in the transverse component of the excited magnetization vector. Because two factors contribute to the decay of transverse magnetization, a combined time constant T2* is generally used to represent the two contributions.
One commonly used pulse sequence in MRI systems is known as a spin-echo sequence. In its traditional form, a ninety degree (90xc2x0) RF pulse is first applied to the spins, as discussed with respect to FIG. 2. Because the spins are all in slightly different environments, the transverse magnetic field begins to dephase. During this dephasing period, a one hundred eighty degree (180xc2x0) pulse is applied. This pulse causes the transverse magnetic field to partially rephase such that a signal is produced called an echo, which is a function of both time constants T1 and T2*. Alternatively, other sequences may be used, such as a gradient echo sequence, a gradient refocused echo sequence, as are well known to those skilled in the art.
To image the spins within the slice created by the slice or Z-axis gradient discussed earlier, two additional gradients are typically applied. The first gradient, called the phase encoding gradient, is applied along one of the sides of the image plane, i.e., conventionally denoted to be on the Y-axis (phase encode axis). Once the phase encoding gradient is turned off, the second gradient, called the frequency encoding gradient, is applied along the remaining edge of the image plane, conventionally denoted to be on the X-axis (readout axis).
Specific thermal imaging pulse sequences have been developed for imaging temperature characteristics within tissue. These sequences have generally used either the temperature sensitivity of the T1 relaxation process, diffusivity contrast, or the shift in the proton resonance peak with respect to temperature. There is interest in applying MRI-guided focused ultrasound to ablate tumors in tissues, such as breast tissue, which contain considerable amounts of fat overlying the pectoral muscle.
Accordingly, it would be desirable to provide systems and methods for imaging a body region containing water-based tissue, such as muscle, as well as fat tissue, using an MR imaging system.
The present invention is directed to systems and methods for imaging tissue using magnetic resonance imaging, and more particularly to methods for performing thermal-sensitive imaging of both fat and muscle tissue using focused magnetic resonance imaging.
In accordance with one aspect of the present invention, a method for thermal magnetic resonance imaging is provided that includes generating a first two echo gradient echo sequence before heating a tissue region within a patient, the first two echo sequence having a first echo optimized for temperature contrast in amplitude from fat tissue and a second echo optimized for temperature phase contrast from water-based tissue. A second two echo gradient echo sequence is generated after heating the tissue region, the second double echo sequence having a third echo optimized for temperature contrast in amplitude from fat tissue and a fourth echo optimized for temperature phase contrast from water-based tissue. Thus, a single RF excitation pulse during each of the two echo sequences may be used to generate the respective pairs of echoes.
The first echo is compared to the third echo to obtain a temperature shift representative of fat tissue, and the second echo is compared to the fourth echo to obtain a temperature shift representative of water-based tissue. In a preferred embodiment, a magnitude difference between the third echo and the first echo is measured and correlated to the temperature shift in fat tissue, and a phase difference between the fourth echo and the second echo is measured and correlated to the temperature shift in water-based tissue. A thermal image may then be generated of the tissue region based upon the temperature shifts in fat and water-based tissue.
In accordance with another aspect of the present invention, a method for thermal magnetic resonance imaging is provided that includes generating a first xe2x80x98two echo gradient echo sequence before heating a tissue region within a patient, the first two echo sequence having a first echo optimized for temperature contrast from fat tissue and a second echo optimized for temperature contrast from water-based tissue. Thermal energy is directed towards the tissue region, for example, using a focused ultrasound system. A second two echo sequence is then generated, the second two echo sequence having a third echo optimized for temperature contrast from fat tissue and a fourth echo optimized for temperature contrast from water-based tissue.
The third echo and the first echo may be compared to obtain a first difference, and the fourth echo and the second echo may be compared to obtain a second difference. A complex difference of the first and second differences may then be combined to facilitate identifying a location of the thermal energy. Alternatively, the complex difference may be compared with at least one of the first difference and the second difference to identify whether the location of the thermal energy is within fat tissue or muscle tissue.
In accordance with yet another aspect of the present invention, a method for thermal magnetic resonance imaging is provided that includes applying a first two echo gradient echo sequence to a tissue region of a patient, the first two echo sequence generating a first echo and a subsequent second echo within the tissue region. Thermal energy is directed towards the tissue region, for example, using a focused ultrasound system. A second two echo gradient echo sequence is applied after directing the thermal energy towards the tissue region, the second two echo sequence generating a third echo and a subsequent fourth echo within the tissue region.
The first echo is compared to the third echo to obtain temperature shift data for fat tissue within the tissue region, and the second echo is compared to the fourth echo to obtain temperature shift data for water-based tissue within the tissue region. Preferably, a magnitude difference between the third echo and the first echo is measured that may be correlated to the temperature shift for fat tissue, and a phase difference between the fourth echo and the second echo is measured that may be correlated to the temperature shift for water-based tissue.
A thermal image may then be generated of the tissue region based upon the temperature shift data for both fat and water-based tissue. Alternatively, a complex difference between the temperature shift data for the fat tissue and the water-based tissue may be combined to identify a location of the thermal energy. Thus, the present invention may provide systems and methods that facilitate monitoring a target tissue region, such as tumor, being treated to ensure that the target tissue region is necrosed without damaging surrounding healthy tissue.
Other objects and features of the present invention will become apparent from consideration of the following description taken in conjunction with the accompanying drawings.